The present invention relates to the electrical, electronic, and computer arts, and, more particularly, to methods for forming substrates with both strained and relaxed silicon regions, and substrates formed thereby.
The inclusion of strained channels in modern metal-oxide-semiconductor field effect transistor (MOSFET) devices holds the promise of increased charge carrier mobilities and transistor device currents. The performance of an n-type field effect transistor (NFET) is typically improved if its channel includes biaxially strained silicon with tensile strain.
Because of lattice mismatches, tensily strained silicon may be formed by epitaxially growing silicon on relaxed silicon-germanium. Such a technique is, for example, utilized when forming wafers by Strained-Si-Directly-on-Insulator (SSDOI) fabrication methodologies. During SSDOI fabrication, a strained silicon layer is typically grown on a silicon-germanium buffer layer of about 1-2 micrometers thickness. Subsequent processing, which includes wafer bonding to a thermally oxidized handle wafer, provides a wafer entirely covered in a strained silicon layer that overlies a thermal oxide layer.